Since bandwidth is a limited resource, there is always a need for more efficient and flexible use of communications bandwidth, and this is particularly so for radio communications. Traditionally, bandwidth is allocated in fixed frequency bands that are assigned operators and users. Recent communications systems have introduced variable bandwidth transmitters and receivers so that the bandwidth can be allocated in amounts as needed. To accommodate this variable bandwidth flexibility, receivers must be able to receive signals over a large frequency band.
But there are problems with wideband receiving. One problem is so-called blocking signals. Blocking signals are signals received a relatively high amplitude that cause one or more components in the receiver to saturate, malfunction, or operate in an undesirable fashion. For example, a blocking signal may saturate one or more amplifiers in the receiver chain or an analog-to-digital converter for digital communications receivers. A blocking signal may only have a narrow frequency band, but nevertheless, its high amplitude can saturate and overpower the entire operation of the wideband receiver. The wider the frequency over which the bandwidth can vary, the more likely that a blocking signal will be received and the receiver overloaded.
Another problem with wideband receiving relates to the initial bandlimiting filter typically used in receivers. A bandpass filter passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. In practice, no bandpass filter is ideal, and thus, does not attenuate all frequencies outside the desired frequency range completely. In particular, there is a region just outside the intended pass-band where frequencies are attenuated, but not sufficiently rejected. This is known as the filter roll-off, and is usually expressed in decibels (dB) of attenutation per octave of frequency. Filter designs seek to make the roll-off as steep as possible—sometimes referred to as a filter with a high Q or quality factor—and thus for the filter to perform as close as possible to its design. But as the roll-off is narrowed, the passband is no longer flat and begins to “ripple.” The steeper the filter roll-off, the more selective the filter.
A steeper filter roll-off becomes more and more difficult to achieve as the width of the passband increases. The result is that a wider bandwidth bandpass filter must use a lower percentage of the allocated bandwidth as compared to a narrower bandwidth bandpass filter. For example, a 1 MHz bandpass has a steeper filter roll-off “skirt” than a 5 MHz bandpass filter for the same number of poles. FIGS. 1A and 1B illustrate this phenomena. The 1 MHz bandpass filter in FIG. 1A passes one fifth of the 5 MHz filter. The roll-off skirt on either side of the passband in the 1 MHz filter is about 0.12 MHz. Information cannot be reliably received in these filter skirt bands, and as a result, 0.24 MHz of 1 MHz bandwidth cannot be used and is wasted. The 5 MHz bandpass filter in FIG. 1B only passes 3.84 MHz. The roll-off skirt on either side of the passband is 0.58 MHz. As a result, 1.16 MHz of bandwidth cannot be used, and about five times as much bandwidth is wasted as compared to the 1 MHz filter.
But combining multiple narrow bandpass filters to approximate a wider bandpass is also problematic. Even using steeper filter skirts, there is still significant unuseable bandwidith. This is because a sufficient guard band normally must be provided between adjacent frequency bands to prevent them from interfering with each other. The guard band is usually the same or more bandwidth than the bandwidth of the filter's roll-off skirt. Two adjacent 5 MHz frequency bands are shown in FIG. 2. Neither the filter passbands nor the filter skirts are permitted to overlap. Consequently, the guard band between the two fields is twice the bandwidth of one side of the filter skirt or 1.16 MHz. To make matters worse, communications specifications also often require that an even greater distance offset distance than the guard band be maintained from a closest blocking signal to the center frequency of the pass band. In FIG. 2, the example blocking signal offset is shown as 2.9 MHz. As a result of all these various limitations, only 7.68 MHz out of 10 MHz in this example of FIG. 2 can actually be used for communications.